Shift means of the above stated kind are known wherein the servo-drives are pneumatic or hydraulic operating cylinders. Particularly in those cases where each sliding gear block can occupy more than two positions, the design and fitting of such a means are very complicated and therefore very expensive. Since each pneumatic or hydraulic operating cylinder can occupy only two specific positions and can therefore bring the sliding gear blocks into only two positions, a further operating cylinder has to be provided for each of the additional positions of the sliding gear blocks.
The object of the present invention is to provide a shift means of the initially stated kind which is simple in construction and inexpensive to produce, and which requires little space.
According to the invention, this object is achieved in that each servo-drive comprises an electric servo-motor equipped with a gear reduction unit and a planar cam drive which is arranged between said servo-motor and the transmission means and has a swinging output lever, the two end positions of the output lever corresponding to the end positions of the sliding gear block.
Electric servo-motors are among the very simplest of servo-drives. Since they are used in large numbers in other technical fields (e.g. as windshield-wiper motors in automobiles), they have been very fully developed and are inexpensive. The gear reduction unit connected downstream, on the one hand, increases the resolution of the entire servo-drive in relation to gear reduction and, on the other hand, also permits the use of small motors having relatively low torque, so that the space required for accommodating the entire unit can be kept small. A further advantage over pneumatic and hydraulic drives, in particular, is that electric servo-motors are able to occupy not just two specific shift positions, but any required number of readily reached intermediate positions.
By means of the cam drive the rotary movement of the electric servo-motor is converted into a reciprocatory movement as finally needed for displacing the sliding gear blocks. A cam drive also represents a simple and inexpensive component which occupies little space.
In accordance with one feature of the invention, the cam drive comprises a cam disc having a closed control curve. Thus, use can be made of a servo-motor having only one direction of rotation, which motor can be held in a very simple manner particularly at its control part. With this arrangement, any position of the shift means can be approached in one direction of rotation, since all the shift positions are periodically repeated during one revolution of the cam disc. According to a further feature of the invention and for the purpose of displacing a sliding gear block having two shift positions, use can be made of a cam disc, the control curve of which takes the form of a solid body or triangular eccentric.
In accordance with yet another feature of the invention, the cam disc has an open control curve, and the servo-motor must then be designed to move to the right and to the left.
According to a still further feature of the invention, the control curves each have, in the zones corresponding to an engagement position of the sliding gear blocks, portions which extend at least substantially concentrically in relation to the axis of rotation of the cam disc. The angular position of the output lever does not change in these zones even when the cam disc still rotates to a slight extent. In this way it becomes possible to dispense with a precise and angularly accurate stoppage of the servo-motor in its shift positions, since small inaccuracies in the angular position of the cam disc in the zone of the shift position do not affect the angular position of the output lever.
The cam disc can be provided with a control curve in the form of a channel in which a guide shaft or the like, arranged on the output lever, is guided on a shape-locking basis, or the cam disc may have a peripheral shape which forms the control curve and against which a guide roller arranged on the output shaft bears in a mechanically positive manner.
In a further embodiment of the invention, a planar crank drive with a swinging output lever is arranged between the electric servo-motor, provided with a reduction gear, and the transmission means, the two end positions of the sliding gear block corresponding to the end positions of the output lever. The crank drive is preferably designed as a crank oscillator which comes to a stop approximately in the zone of the end positions of the output lever.
In accordance with yet another feature of the invention, the servo-motors are controlled by a manually operated preselection switch for feeding in the required values, by cam discs which are each driven by the servo-motor itself and signal particular actual positions of the sliding gear blocks by way of limit switches, and by a comparator for comparing the required and actual positions and for switching off the servo-motors when the preselected gear position is reached.